Process for preparing high vinyl polbutadiene rubber

ABSTRACT

This invention reveals a process for preparing high vinyl polybutadiene rubber which comprises polymerizing 1,3-butadiene monomer with a lithium initiator at a temperature which is within the range of about 5° C. to about 100° in the presence of a sodium alkoxide and a polar modifier, wherein the molar ratio of the sodium alkoxide to the polar modifier is within the range of about 0.1:1 to about 10:1; and wherein the molar ratio of the sodium alkoxide to the lithium initiator is within the range of about 0.05:1 to about 10:1.

This is a divisional of application Ser. No. 08/531,841, filed Sep. 22,1995 now issued as U.S. Pat. No. 5,534,592.

BACKGROUND OF THE INVENTION

It is highly desirable for tires to exhibit good tractioncharacteristics on both dry and wet surfaces. However, it hastraditionally been very difficult to improve the tractioncharacteristics of a tire without compromising its rolling resistanceand tread wear. Low rolling resistance is important because good fueleconomy is virtually always an important consideration. Good tread wearis also an important consideration because it is generally the mostimportant factor which determines the life of the tire.

The traction, tread wear, and rolling resistance of a tire is dependentto a large extent on the dynamic viscoelastic properties of theelastomers utilized in making the tire tread. In order to reduce therolling resistance of a tire, rubbers having a high rebound havetraditionally been utilized in making the tire's tread. On the otherhand, in order to increase the wet skid resistance of a tire, rubberswhich undergo a large energy loss have generally been utilized in thetire's tread. In order to balance these two viscoelasticallyinconsistent properties, mixtures of various types of synthetic andnatural rubber are normally utilized in tire treads. For instancevarious mixtures of styrene-butadiene rubber and polybutadiene rubberare commonly used as a rubber material for automobile tire treads.However, such blends are not totally satisfactory for all purposes.

The inclusion of styrene-butadiene rubber (SBR) in tire treadformulations can significantly improve the traction characteristics oftires made therewith. However, styrene is a relatively expensive monomerand the inclusion of SBR is tire tread formulations leads to increasedcosts.

Carbon black is generally included in rubber compositions which areemployed in making tires and most other rubber articles. It is desirableto attain the best possible dispersion of the carbon black throughoutthe rubber to attain optimized properties. It is also highly desirableto improve the interaction between the carbon black and the rubber. Byimproving the affinity of the rubber compound to the carbon black,physical properties can be improved. Silica can also be included in tiretread formulations to improve rolling resistance.

U.S. Pat. No. 4,843,120 discloses that tires having improved performancecharacteristics can be prepared by utilizing rubbery polymers havingmultiple glass transition temperatures as the tread rubber. Theserubbery polymers having multiple glass transition temperatures exhibit afirst glass transition temperature which is within the range of about-110° C. to -20° C. and exhibit a second glass transition temperaturewhich is within the range of about -50° C. to 0° C. According to U.S.Pat. No. 4,843,120, these polymers are made by polymerizing at least oneconjugated diolefin monomer in a first reaction zone at a temperatureand under conditions sufficient to produce a first polymeric segmenthaving a glass transition temperature which is between -110° C. and -20°C. and subsequently continuing said polymerization in a second reactionzone at a temperature and under conditions sufficient to produce asecond polymeric segment having a glass transition temperature which isbetween -20° C. and 20° C. Such polymerizations are normally catalyzedwith an organolithium catalyst and are normally carried out in an inertorganic solvent.

U.S. Pat. No. 5,137,998 discloses a process for preparing a rubberyterpolymer of styrene, isoprene, and butadiene having multiple glasstransition temperatures and having an excellent combination ofproperties for use in making tire treads which comprises:terpolymerizing styrene, isoprene and 1,3-butadiene in an organicsolvent at a temperature of no more than about 40° C. in the presence of(a) at least one member selected from the group consisting oftripiperidino phosphine oxide and alkali metal alkoxides and (b) anorganolithium compound.

U.S. Pat. No. 5,047,483 discloses a pneumatic tire having an outercircumferential tread where said tread is a sulfur cured rubbercomposition comprised of, based on 100 parts by weight rubber (phr), (A)about 10 to about 90 parts by weight of a styrene, isoprene, butadieneterpolymer rubber (SIBR), and (B) about 70 to about 30 weight percent ofat least one of cis 1,4-polyisoprene rubber and cis 1,4-polybutadienerubber wherein said SIBR rubber is comprised of (1) about 10 to about 35weight percent bound styrene, (2) about 30 to about 50 weight percentbound isoprene and (3) about 30 to about 40 weight percent boundbutadiene and is characterized by having a single glass transitiontemperature (Tg) which is in the range of about -10° C. to about -40° C.and, further the said bound butadiene structure contains about 30 toabout 40 percent 1,2-vinyl units, the said bound isoprene structurecontains about 10 to about 30 percent 3,4-units, and the sum of thepercent 1,2-vinyl units of the bound butadiene and the percent 3,4-unitsof the bound isoprene is in the range of about 40 to about 70 percent.

U.S. Pat. No. 5,272,220 discloses a styrene-isoprene-butadiene rubberwhich is particularly valuable for use in making truck tire treads whichexhibit improved rolling resistance and tread wear characteristics ,said rubber being comprised of repeat units which are derived from about5 weight percent to about 20 weight percent styrene, from about 7 weightpercent to about 35 weight percent isoprene, and from about 55 weightpercent to about 88 weight percent 1,3-butadiene, wherein the repeatunits derived from styrene, isoprene and 1,3-butadiene are inessentially random order, wherein from about 25% to about 40% of therepeat units derived from the 1,3-butadiene are of thecis-microstructure, wherein from about 40% to about 60% of the repeatunits derived from the 1,3-butadiene are of the trans-microstructure,wherein from about 5% to about 25% of the repeat units derived from the1,3-butadiene are of the vinyl-microstructure, wherein from about 75% toabout 90% of the repeat units derived from the isoprene are of the1,4-microstructure, wherein from about 10% to about 25% of the repeatunits derived from the isoprene are of the 3,4-microstructure, whereinthe rubber has a glass transition temperature which is within the rangeof about -90° C. to about -70° C., wherein the rubber has a numberaverage molecular weight which is within the range of about 150,000 toabout 400,000, wherein the rubber has a weight average molecular weightof about 300,000 to about 800,000, and wherein the rubber has aninhomogeneity which is within the range of about 0.5 to about 1.5.

U.S. Pat. No. 5,239,009 reveals a process for preparing a rubberypolymer which comprises: (a) polymerizing a conjugated diene monomerwith a lithium initiator in the substantial absence of polar modifiersat a temperature which is within the range of about 5° C. to about 100°C. to produce a living polydiene segment having a number averagemolecular weight which is within the range of about 25,000 to about350,000; and (b) utilizing the living polydiene segment to initiate theterpolymerization of 1,3-butadiene, isoprene, and styrene, wherein theterpolymerization is conducted in the presence of at least one polarmodifier at a temperature which is within the range of about 5° C. toabout 70° C. to produce a final segment which is comprised of repeatunits which are derived from 1,3-butadiene, isoprene, and styrene,wherein the final segment has a number average molecular weight which iswithin the range of about 25,000 to about 350,000. The rubbery polymermade by this process is reported to be useful for improving the wet skidresistance and traction characteristics of tires without sacrificingtread wear or rolling resistance.

U.S. Pat. No. 5,061,765 discloses isoprene-butadiene copolymers havinghigh vinyl contents which can reportedly be employed in building tireswhich have improved traction, rolling resistance, and abrasionresistance. These high vinyl isoprene-butadiene rubbers are synthesizedby copolymerizing 1,3-butadiene monomer and isoprene monomer in anorganic solvent at a temperature which is within the range of about -10°C. to about 100° C. in the presence of a catalyst system which iscomprised of (a) an organoiron compound, (b) an organoaluminum compound,(c) a chelating aromatic amine, and (d) a protonic compound; wherein themolar ratio of the chelating amine to the organoiron compound is withinthe range of about 0.1:1 to about 1:1, wherein the molar ratio of theorganoaluminum compound to the organoiron compound is within the rangeof about 5:1 to about 200:1, and herein the molar ratio of the protoniccompound to the organoaluminum compound is within the range of about0.001:1 to about 0.2:1.

U.S. Pat. No. 5,405,921 discloses an isoprene-butadiene rubber which isparticularly valuable for use in making truck tire treads, said rubberbeing comprised of repeat units which are derived from about 20 weightpercent to about 50 weight percent isoprene and from about 50 weightpercent to about 80 weight percent 1,3-butadiene, wherein the repeatunits derived from isoprene and 1,3-butadiene are in essentially randomorder, wherein from about 3% to about 10% of the repeat units in saidrubber are 1,2-polybutadiene units, wherein from about 50% to about 70%of the repeat units in said rubber are 1,4-polybutadiene units, whereinfrom about 1% to about 4% of the repeat units in said rubber are3,4-polyisoprene units, wherein from about 25% to about 40% of therepeat units in the polymer are 1,4-polyisoprene units, wherein therubber has a glass transition temperature which is within the range ofabout -90° C. to about -75° C., and wherein the rubber has a Mooneyviscosity which is within the range of about 55 to about 140.

SUMMARY OF THE INVENTION

The high performance polybutadiene rubber blends of this invention haveexcellent properties for incorporation into the rubber compounds used inmaking tire treads. The polybutadiene blends of this invention can beemployed to improve the wet skid resistance and traction characteristicsof tires without sacrificing tread wear or rolling resistance. A highlydesirable combination of tire characteristics can be attained withoututilizing styrene-butadiene rubber.

The present invention more specifically discloses a high performancepolybutadiene rubber blend for utilization in manufacturing tire treadcompounds which is comprised of (a) a first polybutadiene rubber, (b) asecond polybutadiene rubber, and (c) a third polybutadiene rubber;wherein the first polybutadiene rubber, the second polybutadiene rubber,and the third polybutadiene rubber are selected from the group ofconsisting of super-high vinyl polybutadiene rubbers having a vinylcontent which is within the range of 80 percent to 100 percent, highvinyl polybutadiene rubbers having a vinyl content which is within therange of 60 percent to 79 percent, medium vinyl polybutadiene rubbershaving a vinyl content which is within the range of 35 percent to 59percent, and low vinyl polybutadiene rubbers having a vinyl contentwhich is within the range of 8 percent to 34 percent; wherein at leastthree members selected from the group consisting of super-high vinylpolybutadiene rubbers, high vinyl polybutadiene rubbers, medium vinylpolybutadiene rubbers, and low vinyl polybutadiene rubbers are includedin the blend, wherein at least one of the polybutadiene rubbers in theblend has a vinyl content which is within 35 percentage points of thevinyl content of at least one other polybutadiene rubber in the blend,wherein the vinyl content of the first polybutadiene rubber differs fromthe vinyl content of the second polybutadiene rubber and the thirdpolybutadiene rubber by at least 5 percentage points, wherein the vinylcontent of the second polybutadiene rubber differs from the vinylcontent of the third polybutadiene rubber by at least 5 percentagepoints, wherein the blend as a whole has a total vinyl content of atleast 40 percent, and wherein the blend contains at least 10 parts ofthe first polybutadiene rubber, at least 10 parts of the secondpolybutadiene rubber and at least 10 parts of the third polybutadienerubber, based upon a total of 100 parts of rubber in the blend.

The various polybutadiene rubbers employed in the blends of thisinvention can be made utilizing the techniques of this invention toattain various desired vinyl contents. The subject invention furtherdiscloses a process for preparing high vinyl polybutadiene rubber whichcomprises: polymerizing 1,3-butadiene monomer with a lithium initiatorat a temperature which is within the range of about 5° C. to about 100°C. in the presence of a sodium alkoxide and a polar modifier, whereinthe molar ratio of the sodium alkoxide to the polar modifier is withinthe range of about 0.1:1 to about 10:1; and wherein the molar ratio ofthe sodium alkoxide to the lithium initiator is within the range ofabout 0.05:1 to about 10:1.

The subject invention also specifically discloses an initiator systemwhich is comprised of (a) a lithium initiator, (b) a sodium alkoxide,and (c) a polar modifier; wherein the molar ratio of the sodium alkoxideto the polar modifier is within the range of about 0.1:1 to about 10:1;and wherein the molar ratio of the sodium alkoxide to the lithiuminitiator is within the range of about 0.01:1 to about 20:1.

DETAILED DESCRIPTION OF THE INVENTION

The super-high vinyl polybutadiene rubbers, high vinyl polybutadienerubbers, medium vinyl polybutadiene rubbers, and low vinyl polybutadienerubbers which can be employed in the blends of this invention aretypically synthesized by anionic polymerization in an organic medium.The polymerization is normally carried out in an inert organic mediumutilizing a lithium catalyst. The vinyl content of the polybutadienerubber made is controlled by the amount of modifier system presentduring the polymerization step.

The inert organic medium which is utilized as the solvent will typicallybe a hydrocarbon which is liquid at ambient temperatures which can beone or more aromatic, paraffinic or cycloparaffinic compounds. Thesesolvents will normally contain from 4 to 10 carbon atoms per moleculeand will be liquids under the conditions of the polymerization. It is,of course, important for the solvent selected to be inert. The term"inert" as used herein means that the solvent does not interfere withthe polymerization reaction or react with the polymers made thereby.Some representative examples of suitable organic solvents includepentane, isooctane, cyclohexane, normal hexane, benzene, toluene,xylene, ethylbenzene and the like, alone or in admixture. Saturatedaliphatic solvents, such as cyclohexane and normal hexane, are mostpreferred.

The lithium catalysts which can be used are typically organolithiumcompounds. The organolithium compounds which are preferred can berepresented by the formula: R--Li, wherein R represents a hydrocarbylradical containing from 1 to about 20 carbon atoms. Generally, suchmonofunctional organolithium compounds will contain from 1 to about 10carbon atoms. Some representative examples of organolithium compoundswhich can be employed include methyllithium, ethyllithium,isopropyllithium, n-butyllithium, sec-butyllithium, n-octyllithium,tert-octyllithium, n-decyllithium, phenyllithium, 1-napthyllithium,4-butylphenyllithium, p-tolyllithium, 1-naphthyllithium,4-butylphenyllithium, p-tolyllithium, 4-phenylbutyllithium,cyclohexyllithium, 4-butylcyclohexyllithium, and4-cyclohexylbutyllithium. Organo monolithium compounds, such asalkyllithium compounds and aryllithium compounds, are usually employed.Some representative examples of preferred organo monolithium compoundsthat can be utilized include-ethyllithium, isopropyllithium,n-butyllithium, secondary-butyllithium, normal-hexyllithium,tertiary-octyllithium, phenyllithium, 2-napthyllithium,4-butylphenyllithium, 4-phenylbutyllithium, cyclohexyllithium, and thelike. Normal-butyllithium and secondary-butyllithium are highlypreferred lithium initiators.

The amount of lithium catalyst utilized will vary from one organolithiumcompound to another and with the molecular weight that is desired forthe polybutadiene rubber being synthesized. As a general rule in allanionic polymerizations, the molecular weight (Mooney viscosity) of thepolymer produced is inversely proportional to the amount of catalystutilized. As a general rule, from about 0.01 phm (parts per hundredparts by weight of monomer) to 1 phm of the lithium catalyst will beemployed. In most cases, from 0.01 phm to 0.1 phm of the lithiumcatalyst will be employed with it being preferred to utilize 0.025 phmto 0.07 phm of the lithium catalyst.

Normally, from about 5 weight percent to about 35 weight percent of the1,3-butadiene monomer will be charged into the polymerization medium(based upon the total weight of the polymerization medium including theorganic solvent and monomer). In most cases, it will be preferred forthe polymerization medium to contain from about 10 weight percent toabout 30 weight percent monomer. It is typically more preferred for thepolymerization medium to contain from about 20 weight percent to about25 weight percent monomer.

The 1,3-butadiene will be polymerized at a temperature which is withinthe range of about 5° C. to about 100° C. The polymerization temperaturewill preferably be within the range of about 40° C. to about 90° C. forpractical reasons and to attain the desired microstructure. Temperatureswithin the range of about 60° C. to about 80° C. are most preferred. Themicrostructure of the polybutadiene rubber being prepared is somewhatdependent upon the polymerization temperature.

The polymerization is allowed to continue until essentially all of the1,3-butadiene monomer has been exhausted. In other words, thepolymerization is allowed to run to completion. Since a lithium catalystis employed to polymerize the 1,3-butadiene monomer, a livingpolybutadiene rubber is produced. The living polybutadiene rubbersynthesized will have a number average molecular weight which is withinthe range of about 25,000 to about 700,000. The polybutadiene rubbersynthesized will more typically have a number average molecular weightwhich is within the range of about 150,000 to about 400,000.

To increase the level of vinyl content the polymerization can be carriedout in the presence of at least one polar modifier. Ethers and tertiaryamaines which act as Lewis bases are representative examples of polarmodifiers that can be utilized. Some specific examples of typical polarmodifiers include diethyl ether, di-n-propyl ether, diisopropyl ether,di-n-butyl ether, tetrahydrofuran, dioxane, ethylene glycol dimethylether, ethylene glycol diethyl ether, diethylene glycol dimethyl ether,diethylene glycol diethyl ether, triethylene glycol dimethyl ether,trimethylamine, triethylamine, N, N, N', N'- tetramethylethylenediamine,N-methyl morpholine, N-ethyl morpholine, N-phenyl morpholine and thelike.

The modifier can also be a 1,2,3-trialkoxybenzene or a1,2,4-trialkoxybenzene. Some representative examples of1,2,3-trialkoxybenzenes that can be used include1,2,3-trimethoxybenzene, 1,2,3-triethoxybenzene, 1,2,3-tributoxybenzene,1,2,3-trihexoxybenzene, 4,5,6-trimethyl-1,2,3-trimethoxybenzene,4,5,6-tri-n-pentyl-1,2,3-triethoxybenzene,5-methyl-1,2,3-trimethoxybenzene, and 5-propyl-1,2,3-trimethoxybenzene.Some representative examples of 1,2,4-trialkoxybenzenes that can be usedinclude 1,2,4-trimethoxybenzene, 1,2,4-triethoxybenzene,1,2,4-tributoxybenzene, 1,2,4-tripentoxybenzene,3,5,6-trimethyl-1,2,4-trimethoxybenzene,5-propyl-1,2,4-trimethoxybenzene, and3,5-dimethyl-1,2,4-trimethoxybenzene. Dipiperidinoethane,dipyrrolidinoethane, tetramethylethylene diamine, diethylene glycol,dimethyl ether and tetrahydrofuran are representative of highlypreferred modifiers. U.S. Pat. No. 4,022,959 describes the use of ethersand tertiary amines as polar modifiers in greater detail.

The utilization of 1,2,3-trialkoxybenzenes and 1,2,4-trialkoxybenzenesas modifiers is described in greater detail in U.S. Pat. No. 4,696,986.The teachings of U.S. Pat. No. 4,022,959 and U.S. Pat. No. 4,696,986 areincorporated herein by reference in their entirety. The microstructureof the repeat units which are derived from butadiene monomer is afunction of the polymerization temperature and the amount of polarmodifier present. For example, it is known that higher temperaturesresult in lower vinyl contents (lower levels of 1,2-microstructure).Accordingly, the polymerization temperature, quantity of modifier andspecific modifier selected will be determined with the ultimate desiredmicrostructure of the polybutadiene rubber being synthesized being keptin mind.

In cases where super-high vinyl polybutadiene rubber is beingsynthesized a combination of a sodium alkoxide and polar modifier willnormally be employed as the modifier system. It has been unexpectedlyfound that a combination of a sodium alkoxide and polar modifier actssynergistically to increase the vinyl content of polybutadiene rubbersynthesized in its presence. The utilization of this synergisticmodifier system can also be employed advantageously in the synthesis ofhigh vinyl polybutadiene rubber.

The sodium alkoxides which can be utilized in the catalyst system willnormally be of the formula NaOR, wherein R is an alkyl group containingfrom about 2 to about 12 carbon atoms. The sodium metal alkoxide willtypically contain from about 2 to about 12 carbon atoms. It is generallypreferred for the sodium alkoxide to contain from about 3 to about 8carbon atoms. It is generally most preferred for the sodium alkoxide tocontain from about 4 to about 6 carbon atoms. Sodium t-amyloxide (sodiumt-pentoxide) is a representative example of a preferred sodium alkoxideswhich can be utilized in the modifier systems of this invention.

The molar ratio of the sodium alkoxide to the polar modifier in themodifier system will normally be within the range of about 0.1:1 toabout 10:1 and the molar ratio of the sodium alkoxide to the lithiuminitiator will normally be within the range of about 0.01:1 to about20:1. It is generally preferred for the molar ratio of the sodiumalkoxide to the polar modifier to be within the range of about 0.2:1 toabout 5:1 and for the molar ratio of the sodium alkoxide to the lithiuminitiator to be within the range of about 0.05:1 to about 10:1. It isgenerally more preferred for the molar ratio of the sodium alkoxide tothe polar modifier to be within the range of about 0.5:1 to about 1:1and for the molar ratio of the sodium alkoxide to the lithium initiatorto be within the range of about 0.2:1 to about 3:1.

After the polymerization has been completed, the polybutadiene rubbercan be recovered from the organic solvent. The polybutadiene rubber canbe recovered from the organic solvent and residue by any means, such asdecantation, filtration, centrification and the like. It is oftendesirable to precipitate the polybutadiene rubber from the organicsolvent by the addition of lower alcohols containing from about 1 toabout 4 carbon atoms to the polymer solution. Suitable lower alcoholsfor precipitation of the polybutadiene rubber from the polymer cementinclude methanol, ethanol, isopropyl alcohol, normal-propyl alcohol andt-butyl alcohol. The utilization of lower alcohols to precipitate thepolybutadiene rubber from the polymer cement also "kills" the livingpolymer by inactivating lithium end groups. After the polybutadienerubber is recovered from the solution, steam stripping can be employedto reduce the level of volatile organic compounds in the polymer.

There are valuable benefits associated with utilizing the specificpolybutadiene rubber blend of this invention in making tire treadcompounds. Tire tread compounds can be made using only the polybutadienerubbers of this invention without the need to blend additional rubbers,such as styrene-butadiene rubber, therein.

The high performance polybutadiene rubber blends of this inventioncontain at least three different polybutadiene rubbers having differentvinyl contents. The blend can include: (1) super-high vinylpolybutadiene rubber which has a vinyl content which is within the rangeof 80 percent to 100 percent and a glass transition temperature which iswithin the range of about -15° C. to about 0° C., (2) high vinylpolybutadiene rubber which has a vinyl content which is within the rangeof 60 percent to 79 percent and a glass transition temperature which iswithin the range of about -45° C. to about -40° C., (3) medium vinylpolybutadiene rubber which has a vinyl content which is within the rangeof 35 percent to 59 percent and a glass transition temperature which iswithin the range of about -75° C. to about -45° C., and (4) low vinylpolybutadiene rubber which has a vinyl content which is within the rangeof 8 percent to 34 percent and a glass transition temperature which iswithin the range of about -95° C. to about -75° C. However, it iscritical for the blend to contain at least three of the four members ofthe group consisting of super-high vinyl polybutadiene rubber, highvinyl polybutadiene rubber, medium vinyl polybutadiene rubber, and lowvinyl polybutadiene rubber.

It is also critical for at least one of the polybutadiene rubbers in theblend to have a vinyl content which is within 35 percentage points ofthe vinyl content of at least one other polybutadiene rubber in theblend to provide compatibility. It is preferred for at least one of thepolybutadiene rubbers in the blend to have a vinyl content which iswithin 30 percentage points of the vinyl content of at least one otherpolybutadiene rubber in the blend. It is more preferred for at least oneof the polybutadiene rubbers in the blend to have a vinyl content whichis within 25 percentage points of the vinyl content of at least oneother polybutadiene rubber in the blend. For instance, it would behighly preferred for the blend to contain a super-high vinylpolybutadiene rubber having a vinyl content of 90 percent and a highvinyl polybutadiene rubber having a vinyl content of 65 percent (thevinyl content of the high vinyl polybutadiene rubber differs from thevinyl content of the super-high vinyl polybutadiene by only 25percentage points).

It is also important for the three different polybutadiene rubbersemployed in the blend to have vinyl contents which differ from the othertwo polybutadiene rubbers employed in the blend by at least 5 percentagepoints. In other words, the vinyl contents of the differentpolybutadiene rubbers utilized in the blend must differ by at least 5percentage points. For example, if a super-high vinyl polybutadienerubber having a vinyl content of 80 percent and a high vinylpolybutadiene rubber are employed in the blend, the vinyl content of thehigh vinyl polybutadiene must be less than 75 percent. It is preferredfor the three different polybutadiene rubbers employed in the blend tohave vinyl contents which differ from the other two polybutadienerubbers employed in the blend by at least 10 percentage points. Thus, itwould be highly preferred to utilize a super-high vinyl polybutadienerubber having a vinyl content of 85 percent and a high vinylpolybutadiene rubber having a vinyl content of 70 percent in the blend(there is a 15 percentage point difference between the vinyl contents ofthe two polybutadiene rubbers. Stated in still another way, the vinylcontent of the first polybutadiene rubber can not have a vinyl contentwhich is within 5 percentage points of the vinyl content of the secondpolybutadiene rubber or the third polybutadiene rubber, and the vinylcontent of the second polybutadiene rubber can not have a vinyl contentwhich is within 5 percentage points of the vinyl content of the thirdpolybutadiene rubber.

It is also important for the blend as a whole to have a total vinylcontent of at least 40 percent and preferably 45 percent. The totalvinyl content of the blend as a whole is the sum of the products of thenumber of parts of each of the polybutadiene rubbers included in theblend and the vinyl contents of those polybutadiene rubbers, with thatsum being divided by the total number of parts of polybutadiene rubberincluded in the blend. For example, if the blend included 40 parts of alow vinyl polybutadiene rubber having a vinyl content of 20 percent, 40parts of a medium vinyl polybutadiene rubber having a vinyl content of40 percent, and 20 parts of a super-high vinyl polybutadiene rubberhaving a vinyl content of 80 percent, the blend as a whole would have atotal vinyl content of 40 percent. In another example, if the blendincluded 20 parts of a low vinyl polybutadiene rubber having a vinylcontent of 30 percent, 40 parts of a high vinyl polybutadiene rubberhaving a vinyl content of 60 percent, and 40 parts of a super-high vinylpolybutadiene rubber having a vinyl content of 90 percent, the blend asa whole would have a total vinyl content of 66 percent.

The blends of this invention will contain at least 10 phr (parts per 100parts by weight of rubber) of the first polybutadiene rubber, at least10 phr of the second polybutadiene rubber, and at least 10 phr of thethird polybutadiene rubber. The blends of this invention will preferablycontain at least 20 phr of the first polybutadiene rubber, at least 20phr of the second polybutadiene rubber, and at least 20 phr of the thirdpolybutadiene rubber. The blends of this invention will more preferablycontain at least 25 phr of the first polybutadiene rubber, at least 25phr of the second polybutadiene rubber, and at least 25 phr of the thirdpolybutadiene rubber.

The polybutadiene rubber blends of this invention can be compoundedutilizing conventional ingredients and standard techniques. Forinstance, the polybutadiene rubber blends will typically be mixed withcarbon black and/or silica, sulfur, fillers, accelerators, oils, waxes,scorch inhibiting agents, and processing aids. In most cases, thepolybutadiene rubber blends will be compounded with sulfur and/or asulfur containing compound, at least one filler, at least oneaccelerator, at least one antidegradant, at least one processing oil,zinc oxide, optionally a tackifier resin, optionally a reinforcingresin, optionally one or more fatty acids, optionally a peptizer andoptionally one or more scorch inhibiting agents. Such blends willnormally contain from about 0.5 to 5 phr (parts per hundred parts ofrubber by weight) of sulfur and/or a sulfur containing compound with 1phr to 2.5 phr being preferred. It may be desirable to utilize insolublesulfur in cases where bloom is a problem.

Normally from 10 to 150 phr of at least one filler will be utilized inthe blend with 30 to 80 phr being preferred. In most cases at least somecarbon black will be utilized in the filler. The filler can, of course,be comprised totally of carbon black. Silica can be included in thefiller to improve tear resistance and heat build up. Clays and/or talccan be included in the filler to reduce cost. The blend will alsonormally include from 0.1 to 2.5 phr of at least one accelerator with0.2 to 1.5 phr being preferred. Antidegradants, such as antioxidants andantiozonants, will generally be included in the tread compound blend inamounts ranging from 0.25 to 10 phr with amounts in the range of 1 to 5phr being preferred. Processing oils will generally be included in theblend in amounts ranging from 2 to 100 phr with amounts ranging from 5to 50 phr being preferred. The polybutadiene blends of this inventionwill also normally contain from 0.5 to 10 phr of zinc oxide with 1 to 5phr being preferred. These blends can optionally contain from 0 to 10phr of tackifier resins, 0 to 10 phr of reinforcing resins, 1 to 10 phrof fatty acids, 0 to 2.5 phr of peptizers, and 0 to 1 phr of scorchinhibiting agents.

To fully realize the total advantages of the blends of this inventionsilica will normally be included in the tread rubber formulation. Theprocessing of the polybutadiene rubber blend is normally conducted inthe presence of a sulfur containing organosilicon compound to realizemaximum benefits. Examples of suitable sulfur containing organosiliconcompounds are of the formula:

    Z-Alk-S.sub.n -Alk-Z                                       (I)

in which Z is selected from the group consisting of ##STR1## where R¹ isan alkyl group of 1 to 4 carbon atoms, cyclohexyl or phenyl; wherein R²is alkoxy of 1 to 8 carbon atoms, or cycloalkoxy of 5 to 8 carbon atoms;and wherein Alk is a divalent hydrocarbon of 1 to 18 carbon atoms and nis an integer of 2 to 8.

Specific examples of sulfur containing organosilicon compounds which maybe used in accordance with the present invention include:3,3'-bis(trimethoxysilylpropyl) disulfide,3,3'-bis(triethoxysilylpropyl) tetrasulfide,3,3'-bis(triethoxysilylpropyl) octasulfide,3,3'-bis(trimethoxysilylpropyl) tetrasulfide,2,2'-bis(triethoxysilylethyl) tetrasulfide,3,3'-bis(trimethoxysilylpropyl) trisulfide,3,3'-bis(triethoxysilylpropyl) trisulfide,3,3'-bis(tributoxysilylpropyl) disulfide,3,3'-bis(trimethoxysilylpropyl) hexasulfide,3,3'-bis(trimethoxysilylpropyl) octasulfide,3,3'-bis(trioctoxysilylpropyl) tetrasulfide,3,3'-bis(trihexoxysilylpropyl) disulfide,3,3'-bis(tri-2"-ethylhexoxysilylpropyl) trisulfide,3,3'-bis(triisooctoxysilylpropyl) tetrasulfide,3,3'-bis(tri-t-butoxysilylpropyl) disulfide, 2,2'-bis(methoxy diethoxysilyl ethyl) tetrasulfide, 2,2'-bis(tripropoxysilylethyl) pentasulfide,3,3'-bis(tricyclonexoxysilylpropyl) tetrasulfide, 3,3'-bis(tricyclopentoxysilylpropyl) trisulfide,2,2'-bis(tri-2"-methylcyclohexoxysilylethyl) tetrasulfide,bis(trimethoxysilylmethyl) tetrasulfide, 3-methoxy ethoxy propoxysilyl3'-diethoxybutoxy- silylpropyltetrasulfide, 2,2'-bis(dimethylmethoxysilylethyl) disulfide, 2,2'-bis(dimethyl sec.butoxysilylethyl)trisulfide, 3,3'-bis(methyl butylethoxysilylpropyl) tetrasulfide,3,3'-bis(di t-butylmethoxysilylpropyl) tetrasulfide, 2,2'-bis(phenylmethyl methoxysilylethyl) trisulfide, 3,3'-bis(diphenylisopropoxysilylpropyl) tetrasulfide, 3,3'-bis(diphenylcyclohexoxysilylpropyl) disulfide, 3,3'-bis(dimethylethylmercaptosilylpropyl) tetrasulfide, 2,2'-bis(methyldimethoxysilylethyl) trisulfide, 2,2'-bis(methylethoxypropoxysilylethyl) tetrasulfide, 3,3'-bis(diethylmethoxysilylpropyl) tetrasulfide, 3,3'-bis(ethyl di-sec.butoxysilylpropyl) disulfide, 3,3'-bis(propyl diethoxysilylpropyl)disulfide, 3,3'-bis(butyl dimethoxysilylpropyl) trisulfide,3,3'-bis(phenyl dimethoxysilylpropyl) tetrasulfide, 3-phenylethoxybutoxysilyl 3'-trimethoxysilylpropyl tetrasulfide,4,4'-bis(trimethoxysilylbutyl) tetrasulfide,6,6'-bis(triethoxysilylhexyl) tetrasulfide,12,12'-bis(triisopropoxysilyl dodecyl) disulfide,18,18'-bis(trimethoxysilyloctadecyl) tetrasulfide,18,18'-bis(tripropoxysilyloctadecenyl) tetrasulfide,4,4'-bis(trimethoxysilyl-buten-2-yl) tetrasulfide,4,4'-bis(trimethoxysilylcyclohexylene) tetrasulfide,5,5'-bis(dimethoxymethylsilylpentyl) trisulfide,3,3'-bis(trimethoxysilyl-2-methylpropyl) tetrasulfide,3,3'-bis(dimethoxyphenylsilyl-2-methylpropyl) disulfide.

The preferred sulfur containing organosilicon compounds are the3,3'-bis(trimethoxy or triethoxy silylpropyl) sulfides. The mostpreferred compound is 3,3'-bis(triethoxysilylpropyl) tetrasulfide.Therefore as to formula I, preferably Z is ##STR2## where R² is analkoxy of 2 to 4 carbon atoms, with 2 carbon atoms being particularlypreferred; Alk is a divalent hydrocarbon of 2 to 4 carbon atoms with 3carbon atoms being particularly preferred; and n is an integer of from 3to 5 with 4 being particularly preferred.

The amount of the sulfur containing organosilicon compound of formula Iin a rubber composition will vary depending on the level of silica thatis used. Generally speaking, the amount of the compound of formula Iwill range from about 0.01 to about 1.0 parts by weight per part byweight of the silica. Preferably, the amount will range from about 0.02to about 0.4 parts by weight per part by weight of the silica. Morepreferably the amount of the compound of formula I will range from about0.05 to about 0.25 parts by weight per part by weight of the silica.

In addition to the sulfur containing organosilicon, the rubbercomposition should contain a sufficient amount of silica, and carbonblack, if used, to contribute a reasonably high modulus and highresistance to tear. The silica filler may be added in amounts rangingfrom about 10 phr to about 250 phr. Preferably, the silica is present inan amount ranging from about 15 phr to about 80 phr. If carbon black isalso present, the amount of carbon black, if used, may vary. Generallyspeaking, the amount of carbon black will vary from about 5 phr to about80 phr. Preferably, the amount of carbon black will range from about 10phr to about 40 phr. It is to be appreciated that the silica coupler maybe used in conjunction with a carbon black, namely pre-mixed with acarbon black prior to addition to the rubber composition, and suchcarbon black is to be included in the aforesaid amount of carbon blackfor the rubber composition formulation. In any case, the total quantityof silica and carbon black will be at least about 30 phr. The combinedweight of the silica and carbon black, as hereinbefore referenced, maybe as low as about 30 phr, but is preferably from about 45 to about 130phr.

The commonly employed siliceous pigments used in rubber compoundingapplications can be used as the silica in this invention, includingpyrogenic and precipitated siliceous pigments (silica), althoughprecipitate silicas are preferred. The siliceous pigments preferablyemployed in this invention are precipitated silicas such as, forexample, those obtained by the acidification of a soluble silicate,e.g., sodium silicate.

Such silicas might be characterized, for example, by having a BETsurface area, as measured using nitrogen gas, preferably in the range ofabout 40 to about 600, and more usually in a range of about 50 to about300 square meters per gram. The BET method of measuring surface area isdescribed in the Journal of the American Chemical Society, Volume 60,page 304 (1930).

The silica may also be typically characterized by having adibutylphthalate (DBP) absorption value in a range of about 100 to about400, and more usually about 150 to about 300. The silica might beexpected to have an average ultimate particle size, for example, in therange of 0.01 to 0.05 micron as determined by the electron microscope,although the silica particles may be even smaller, or possibly larger,in size.

Various commercially available silicas may be considered for use in thisinvention such as, only for example herein, and without limitation,silicas commercially available from PPG Industries under the Hi-Siltrademark with designations 210, 243, etc; silicas available fromRhone-Poulenc, with, for example, designations of Z1165MP and Z165GR andsilicas available from Degussa AG with, for example, designations VN2and VN3.

Tire tread formulations which include silica and an organosiliconcompound will typically be mixed utilizing a thermomechanical mixingtechnique. The mixing of the tire tread rubber formulation can beaccomplished by methods known to those having skill in the rubber mixingart. For example the ingredients are typically mixed in at least twostages, namely at least one non-productive stage followed by aproductive mix stage. The final curatives including sulfur vulcanizingagents are typically mixed in the final stage which is conventionallycalled the "productive" mix stage in which the mixing typically occursat a temperature, or ultimate temperature, lower than the mixtemperature(s) than the preceding non-productive mix stage(s). Therubber, silica and sulfur containing organosilicon, and carbon black ifused, are mixed in one or more non-productive mix stages. The terms"non-productive" and "productive" mix stages are well known to thosehaving skill in the rubber mixing art. The sulfur vulcanizable rubbercomposition containing the sulfur containing organosilicon compound,vulcanizable rubber and generally at least part of the silica should besubjected to a thermomechanical mixing step. The thermomechanical mixingstep generally comprises a mechanical working in a mixer or extruder fora period of time suitable in order to produce a rubber temperaturebetween 140° C. and 190° C. The appropriate duration of thethermomechanical working varies as a function of the operatingconditions and the volume and nature of the components. For example, thethermomechanical working may be for a duration of time which is withinthe range of about 2 minutes to about 20 minutes. It will normally bepreferred for the rubber to reach a temperature which is within therange of about 145° C. to about 180° C. and to be maintained at saidtemperature for a period of time which is within the range of about 4minutes to about 12 minutes. It will normally be more preferred for therubber to reach a temperature which is within the range of about 155° C.to about 170° C. and to be maintained at said temperature for a periodof time which is within the range of about 5 minutes to about 10minutes.

The polybutadiene rubber blend containing tire tread compounds of thisinvention can be used in tire treads in conjunction with ordinary tiremanufacturing techniques. Tires are built utilizing standard procedureswith the polybutadiene rubber blend simply being substituted for therubber compounds typically used as the tread rubber. After the tire hasbeen built with the polybutadiene rubber containing blend, it can bevulcanized using a normal tire cure cycle. Tires made in accordance withthis invention can be cured over a wide temperature range. However, itis generally preferred for the tires of this invention to be cured at atemperature ranging from about 132° C. (270° F.) to about 166° C. (330°F.). It is more typical for the tires of this invention to be cured at atemperature ranging from about 143° C. (290° F.) to about 154° C. (310°F.). It is generally preferred for the cure cycle used to vulcanize thetires of this invention to have a duration of about 10 to about 20minutes with a cure cycle of about 12 to about 18 minutes being mostpreferred.

By utilizing the polybutadiene rubber blends of this invention in tiretread compounds traction characteristics can be improved withoutcompromising tread wear or rolling resistance. Since the polybutadienerubber blends of this invention do not contain styrene the cost of rawmaterials can also be reduced. This is because styrene and other vinylaromatic monomers are expensive relative to the cost of 1,3-butadiene.

This invention is illustrated by the following examples which are merelyfor the purpose of illustration and are not to be regarded as limitingthe scope of the invention or the manner in which it can be practiced.Unless specifically indicated otherwise, all parts and percentages aregiven by weight.

EXAMPLE 1

In this experiment, 1500 g of a silica/alumina/molecular sieve driedpremix containing 12.4 weight percent 1,3-butadiene in hexane wascharged into a one-gallon (3.8 liters) reactor. After the impurity levelof 1.5 ppm was determined, 2.76 ml of 1M solution ofN,N,N',N'-tetramethylethylene diamine (TMEDA) in hexane, 1.84 ml 0.5Msolution of sodium-t-amylate (STA) in hexane and 1.15 ml of a 0.91Msolution of n-butyllithium (in hexane; 1.01 ml for initiation and 0.14ml for scavenging the premix) were added to the reactor. The molar ratioof TMEDA to STA and to n-butyllithium was 3:1:1.

The polymerization was carried out at 75° C. for 10 minutes. The GCanalysis of the residual monomer contained in the polymerization mixtureindicated that the polymerization was complete at this time. Then, threeml of 1M ethanol solution (in hexane) was added to the reactor toshortstop the polymerization and polymer was removed from the reactorand stabilized with 1 phm of antioxidant. After evaporating hexane, theresulting polymer was dried in a vacuum oven at 50° C.

The polybutadiene produced was determined to have a glass transitiontemperature (Tg) at -27° C. It was then determined to have amicrostructure which contained 82 percent 1,2-polybutadiene units and 18percent 1,4-polybutadiene units. The Mooney ML-4 viscosity ML-4 at 100°C. was 54 for this polybutadiene.

EXAMPLES 2-6

The procedure described in Example 1 was utilized in these examplesexcept that the TMEDA/STA/n-butyllithium ratio was varied. The Tgs andmicrostructures of the resulting polybutadienes are listed in Table I.

                  TABLE I                                                         ______________________________________                                             TMEDA/STA/n-BuLi                                                                             Tg     Mooney Microstructure                              Ex.  Ratio          (°C.)                                                                         ML-4   1,2-PBd                                                                             1,4-PBd                               ______________________________________                                        1    3:1:1          -28    54     82    18                                    2    3:2:1          -27    65     83    17                                    3    3:3:1          -27    68     82    18                                    4    3:0.5:1        -28    50     82    18                                    5    2:1:1          -27    50     82    18                                    6    5:1:1          -27    57     83    17                                    ______________________________________                                    

EXAMPLE 7

The procedure described in Example 1 was utilized in this example exceptthat TMEDA was used as the modifier. It took about 60 minutes tocomplete the polymerization. The polymer was determined to have a Tg at-45° C. It was also determined to have a microstructure which contained63 percent 1,2-polybutadiene units and 37 percent 1,4-polybutadieneunits.

EXAMPLE 8

The procedure described in Example 1 was utilized in this example exceptthat TMEDA was used as the modifier and the TMEDA to n-butyl-lithiumratio was changed to 1:1. It took about 75 minutes to complete thepolymerization. The polymer was determined to have a Tg at -63° C. Itwas also determined to have a microstructure which contained 50 percent1,2-polybutadiene units and 50 percent 1,4-polybutadiene units.

EXAMPLE 9-14

The super-high vinyl polybutadiene rubber made in Example 1, the highvinyl polybutadiene rubber made in Example 7, and the medium vinylpolybutadiene rubber made in Example 8 were blended together andcompounded utilizing a standard tire tread test formulation and comparedto tire tread formulations made with a blend of solutionstyrene-butadiene rubber and high cis-1,4-polybutadiene rubber. The tiretread test formulations were made by mixing the ingredients shown inTable II. Example 9 was carried out as a comparative example and withthe rubber component being a blend of solution styrene-butadiene rubberand high cis-1,4-polybutadiene rubber.

                  TABLE II                                                        ______________________________________                                        Example     9     10       11  12     13  14                                  ______________________________________                                        SBR.sup.1   70    --       --  --     --  --                                  Cis-1,4-PBD.sup.2                                                                         30    --       --  --     --  --                                  MVPBD.sup.3 --    50       30  50     30  20                                  HVPBD.sup.4 --    30       50  30     50  70                                  SHVPBD.sup.5                                                                              --    20       20  20     20  10                                  carbon black                                                                              70    70       70  --     --  --                                  Silica      --    --       --  70     70  70                                  Coupling    --    --       --  11     11  11                                  Agent.sup.6                                                                   wax         2     2        2   2      2   2                                   Oil         28    28       28  28     28  28                                  zinc oxide  3.5   3.5      3.5 3.5    3.5 3.5                                 stearic acid                                                                              2     2        2   2      2   2                                   6-PPD.sup.7 1     1        1   1      1   1                                   CBS.sup.8   1.7   1.7      1.7 1.7    1.7 1.7                                 DPG.sup.9   2     2        2   2      2   2                                   sulfur      1.4   1.4      1.4 1.4    1.4 1.4                                 Antioxidant 1     1        1   1      1   1                                   ______________________________________                                         .sup.1  The solution styrenebutadiene rubber contained 10% bound styrene.     .sup.2  The high cis1,4-polybutadiene rubber was Budene ® 1207            polybutadiene rubber.                                                         .sup.3  MVPBD represents medium vinyl polybutadiene rubber having a vinyl     content of about 50 percent.                                                  .sup.4  HVPBD represents high vinyl polybutadiene rubber having a vinyl       content of about 63 percent.                                                  .sup.5  SHVPBD represents superhigh vinyl polybutadiene rubber having a       vinyl content of about 82 percent.                                            .sup.6  The coupling agent was 50 percent active Si69 which was provided      by Degussa as X50S.                                                           .sup.7  6PPD represents N(1,3-dimethyl butyl)Nphenyl-p-phenylenediamine.      .sup.8  CBS represents Ncyclohexyl-2-benzothiazyl-sulfenamide.                .sup.9  DPG represents diphenyl guanidine.                               

The physical properties of the compounded tire tread formulations arereported in Table III.

                  TABLE III                                                       ______________________________________                                        Compound Physical Properties                                                  Example  9       10      11    12    13    14                                 ______________________________________                                        100% Modu-                                                                             1.8     1.7     1.7   2.1   2.3   2.2                                lus, MPa                                                                      300% Modu-                                                                             7.3     7.1     7.1   9.8   9.8   9.4                                lus, MPa                                                                      Break    15.6    15.6    15.3  15.3  15.7  14.7                               Strength, MPa                                                                 Elongation                                                                             580%    587%    577%  438%  459%  443%                               at Break                                                                      Hardness @                                                                             65.7    64.5    64.1  61.6  65.5  64.3                               23° C.                                                                 Hardness @                                                                             54.5    53.2    51.5  55.6  58.7  58.2                               100° C.                                                                Rebound @                                                                              34.3    31.4    30.0  41.6  38.3  38.8                               23° C.                                                                 Rebound @                                                                              50.4    50.7    49.7  64.5  64.0  63.6                               100° C.                                                                ______________________________________                                    

The high rebound at 100° C. reported in Examples 12-14 is indicative ofimproved rolling resistance in tire tread formulations. The lowerrebound at room temperature suggests better traction characteristics.Thus, the tire tread compounds made with silica showed an exceptionalcombination of characteristics for utilization in tire treads.

Variations in the present invention are possible in light of thedescription of it provided herein. It is, therefore, to be understoodthat changes can be made in the particular embodiments described whichwill be within the full intended scope of the invention as defined bythe following appended claims.

What is claimed is:
 1. A process for preparing high vinyl polybutadienerubber which comprises: polymerizing 1,3-butadiene monomer with alithium initiator at a temperature which is within the range of about 5°C. to about 100° C. in the presence of a sodium alkoxide and a polarmodifier, wherein the molar ratio of the sodium alkoxide to the polarmodifier is within the range of about 0.5:1 to about 1:1; and whereinthe molar ratio of the sodium alkoxide to the lithium initiator iswithin the range of about 0.2:1 to about 3:1, wherein the sodiumalkoxide is of the formula NaOR, wherein R represents an alkyl groupcontaining from about 3 to about 8 carbon atoms, wherein said polarmodifier is selected from the group consisting of diethyl ether,di-n-propyl ether, diisopropyl ether, di-n-butyl ether, tetrahydrofuran,dioxane, ethylene glycol dimethyl ether, ethylene glycol diethyl ether,diethylene glycol dimethyl ether, diethylene glycol diethyl ether,triethylene glycol dimethyl ether, trimethylamine, triethylamine,N,N,N',N'-tetramethylethylenediamine, N-methyl morpholine, N-ethylmorpholine, N-phenyl morpholine and alkyltetrahydrofurfuryl ethers.
 2. Aprocess for preparing high vinyl polybutadiene rubber which comprises:polymerizing 1,3-butadiene monomer with a lithium initiator at atemperature which is within the range of about 5° C. to about 100° C. inthe presence of sodium t-pentoxide and a polar modifier, wherein themolar ratio of the sodium t-pentoxide to the polar modifier is withinthe range of about 0.5:1 to about 1:1; and wherein the molar ratio ofthe sodium t-pentoxide to the lithium initiator is within the range ofabout 0.2:1 to about 3:1, wherein said polar modifier isN,N,N',N'-tetramethylethylenediamine.
 3. A process as specified in claim1 wherein the 1,3-butadiene monomer is polymerized at a temperaturewhich is within the range of about 40° C. to about 90° C.
 4. A processas specified in claim 3 wherein the 1,3-butadiene monomer is polymerizedin a polymerization medium which is comprised of an inert organicmedium.
 5. A process as specified in claim 4 wherein the inert organicmedium is a liquid hydrocarbon which contains from 4 to 10 carbon atoms.6. A process as specified in claim 5 wherein the inert organic medium isselected from the group of solvents consisting of pentane, isooctane,cyclohexane, normal hexane, benzene, toluene, xylene and ethylbenzene.7. A process as specified in claim 4 wherein the polymerization mediumcontains from about 10 weight percent to about 30 weight percent1,3-butadiene monomer.
 8. A process as specified in claim 7 wherein the1,3-butadiene monomer is polymerized at a temperature which is withinthe range of about 60° C. to about 80° C.
 9. A process as specified inclaim 1 wherein the lithium initiator is an organolithium compound ofthe formula R--Li, wherein R represents a hydrocarbyl radical containingfrom 1 to about 20 carbon atoms.
 10. A process as specified in claim 1wherein the lithium initiator is selected from the group consisting ofethyllithium, isopropyllithium, n-butyllithium, secondary-butyllithium,n-hexyllithium, tertiary-octyllithium, phenyllithium, 2-napthyllithium,4-butylphenyllithium, 4-phenylbutyllithium and cyclohexyllithium.
 11. Aprocess as specified in claim 1 wherein the lithium initiator isselected from the group consisting of n-butyllithium andsecondary-butyllithium.
 12. A process as specified in claim 1 whereinthe lithium initiator is utilized in an amount which is within the rangeof about 0.1 phm to 0.1 phm.
 13. A process as specified in claim 1wherein the 1,3-butadiene monomer is polymerized in a polymerizationmedium which is comprised of an inert organic medium, and wherein thepolymerization medium contains from about 10 weight percent to about 30weight percent 1,3- butadiene monomer.
 14. A process as specified inclaim 13 wherein the 1,3-butadiene monomer is polymerized at atemperature which is within the range of about 60° C. to about 80° C.15. A process for preparing high vinyl polybutadiene rubber whichcomprises: polymerizing 1,3-butadiene monomer with a lithium initiatorat a temperature which is within the range of about 40° C. to about 90°C. in the presence of a sodium alkoxide and a polar modifier, whereinthe molar ratio of the sodium alkoxide to the polar modifier is withinthe range of about 0.5:1 to about 1:1; and wherein the molar ratio ofthe sodium alkoxide to the lithium initiator is within the range ofabout 0.2:1 to about 3:1, wherein the sodium alkoxide is of the formulaNaOR, wherein R represents an alkyl group containing from about 2 toabout 12 carbon atoms, wherein the 1,3-butadiene monomer is polymerizedin a polymerization medium which is comprised of an inert organicmedium, wherein the organic medium is selected from the group consistingof cyclohexane and normal hexane.
 16. A process as specified in claim 15wherein the lithium initiator is an organolithium compound of theformula R--Li, wherein R represents a hydrocarbyl radical containingfrom 1 to about 20 carbon atoms.
 17. A process as specified in claim 15wherein the lithium initiator is selected from the group consisting ofethyllithium, isopropyllithium, n-butyllithium, secondary-butyllithium,n-hexyllithium, tertiary-octyllithium, phenyllithium, 2-napthyllithium,4-butylphenyllithium, 4-phenylbutyllithium and cyclohexyllithium.
 18. Aprocess as specified in claim 15 wherein the lithium initiator isselected from the group consisting of n-butyllithium andsecondary-butyllithium.
 19. A process as specified in claim 16 whereinthe lithium initiator is utilized in an amount which is within the rangeof about 0.01 phm to 0.1 phm.